OneCare only sees WHS when you first install OneCare. If you have backup turned on already and install a Windows Home Server box and the connector software, you can manually disable OneCare backup.
Open OneCare
Click Change Settings
backup tab
Click change settings in the lower left
Make sure that the first screen for Centralized Backup has "No Centralized backup" selected
Click your PC on the left.
Uncheck the selection for "Turn on backup for this PC."
Click Next and save the revised plan.
-steve